<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>demo1</title>
    <style>
        div {
            width: 300px;
            height: 300px;
        }
        .box {
            margin: 100px auto;
            position: relative;
        }
        .content {
            position: absolute;
            background-color: blue;
            left: -6px;
            top: -6px;
            z-index: 2;
        }
        .mark {
            background-color: black;
            position: absolute;
            right: -6px;
            bottom: -6px;
            z-index: 1;
            opacity: 0.5;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="content"></div>
        <div class="mark"></div>
    </div>
</body>
</html>